Continued development of microprocessor and other integrated circuits introduces new challenges to the development of switching power converters. In order to reduce the passive component size, and also to meet stringent transient response requirements, the switching frequency of power converters will move into the MHz range in the next few years.
In high frequency, low power applications, the effect of the gate driver on the converter efficiency becomes more significant. As the operating frequency of power converters is raised, losses associated with driving the power MOSFET increase in proportion to the switching frequency. At low power levels the resulting penalty on the overall converter efficiency become significant. On the other hand, as power MOSFET die size is increased to reduce MOSFET on-resistance, the gate-source capacitance of the MOSFET increases in a proportional manner. Therefore, in low voltage, high current applications, the gate drive loss will also increase when low on-resistance MOSFETs are chosen to reduce the conduction loss. The gate drive losses can often offset advantages gained by the lower conduction losses.
Hence, lossless gate drive circuits have attracted much attention in recent years. Resonant gate drivers are an efficient alternative to the conventional methods to drive power MOSFETs. Many approaches have already been proposed, most of which are designed for a single MOSFET and are based on L-C resonance techniques (see, for example, “A MOS gate drive with resonant transitions”, D. Maksimovic, IEEE PESC'91, pp. 527-532 (1991); “A resonant MOSFET gate driver with efficient energy recovery”, Y. Chen, F. C. Lee, L. Amoroso, H. Wu, IEEE Transactions on Power Electronics, 19:470-477 (2004); “A resonant power MOSFET/IGBT gate driver”, I. D. de Vries, IEEE, APEC'02, pp. 179-185 (2002)). A simple DC-DC converter and a transformer have been proposed in some solutions (e.g., “A new lossless power MOSFET driver based on simple DC/DC converters”, J. Diaz, M. A. Perez, F. M. Linera, F. Aldana, IEEE PESC'95, pp. 37-43 (1995)). However, this approach makes the gate driver too complicated and limits the energy that can be recovered. In general, the above solutions can only recover limited gate driving loss and provide little other benefit.
In a synchronous buck converter, the switching loss of the high side MOSFET is another restriction which limits the switching frequency, as the switching loss is also proportional to the switching frequency.
A resonant gate driver for two MOSFETs in a synchronous buck converter was proposed by K. Yao and F. C. Lee in “A novel resonant gate driver for high frequency synchronous buck converters,” IEEE Transactions on Power Electronics, 17:180-186 (2003), but the required control signals are difficult to generate, and the coupled inductor is expensive and difficult to design. Further, that solution does not reduce the switching loss of the top MOSFET.
The circuit proposed by Zhang (U.S. Pat. No. 6,441,673, issued Aug. 27, 2002) used a current source to charge the gate capacitance of the power switches. However, in that circuit, charging the gate capacitance with a low initial current resulted in the on-time of the power switch to be limited to a minimum of one-quarter of the resonant period of the L-C circuit, where C is the gate capacitance. This placed a limit on the extent to which switching time and switching loss of the power switch could be reduced.
The derivative of the current (di/dt) is another important issue if the switching frequency is to be increased. MOSFETs may be falsely triggered if the gate driver cannot clamp, or lock, the gate-source voltage of the MOSFET at less than its threshold value while the MOSFET is turned off.
The resonant gate drive circuits of the present invention address the above issues.